Composite board products such as particleboard, medium density fiberboard (MDF), high density fiberboard (HDF), and oriented strand board (OSB) may be used in many applications such as home roofing and flooring, furniture, cabinets, door panels, frames, and signposts, as just a few examples. Generally, to make such composite products a particulate lignocellulosic (wood) substrate is mixed with a binder, and the mixture is put on a mat or other surface for pressing. A press, such as a static or a continuous press, may have one or more platens to exert and/or receive pressure. While in the press, the binder may be activated to bind the substrate particles together. Once cured, the composite product can be removed from the press.
Formaldehyde based binders have dominated the wood composite industry. But formaldehyde is linked to human health risks. For example, it is a common indoor pollutant, it may be toxic, it may cause allergic reactions, and it may be a human carcinogen. Thus, use of formaldehyde in composite wood products may be discouraged.
Isocyanate based binders, particularly polyphenylene polymethylene polyisocyanate (PMDI, or polymeric MDI) binders, offer advantages over formaldehyde based binders. These advantages include improved cure speed, superior board physical and moisture resistance properties, the ability to bond with lignocellulosic materials having high water contents, and the elimination of the formaldehyde emission hazard.
Using polymeric MDI binders in composite wood production does have, however, certain disadvantages. One disadvantage of PMDI binders is binder adhesion to the platen. Another disadvantage of PMDI binders is the accumulation of binder reaction products on the press platen or, when used, a die surface. Build-up on decorative die surfaces can be particularly troubling, as it will eventually accumulate to a point where it will “mask” the decorative surface on the composite product. The die then is un-usable and will have to be removed for cleaning. This cleaning process is costly and significantly reduces productivity.
To overcome these disadvantages, a layered methodology evolved to prevent binder build-up and sticking of pressed board to the press platen. Generally, three layers of binder-treated substrate are used in this method, one core layer, and two surface layers. Polymeric MDI is used to bind the core layer and a different binder, such as a formaldehyde-based binder, is used to bind the two surface layers. The three different layers of binder treated wood particles are laid out on the press prior to curing and care must be taken to prevent isocyanate-treated particles (core layer particles) from breaking through onto the surface layer and contacting the press surface.
Release agents have also been used to prevent PMDI bonded panels from sticking to platen/press surfaces and reducing surface build up (masking). External release agents can be applied to the press surface and/or to the surface of the binder-treated wood particulate mass before pressing takes place. Alternatively or additionally, an internal release agent may be mixed with the binder and/or the bulk particulate mass. Although helpful, release agent technology has not been reliable enough to permit the widespread industrial use of PMDI in surface layers of pressed composite boards.
Anti-bonding agents have also been employed to try to eliminate PMDI-induced adhesion and masking. Generally, a very thin layer of an anti-bonding agent, such as a silane- or silicon-based anti-bonding agent is coated and baked on to the press platen/die surface. In many cases, the anti-bonding agent has resulted in acceptable release, but platen/die masking is still a concern.
Unfortunately, use of release agents and anti-bonding agents have not resulted in widespread use of PMDI based binders in surface layers. Furthermore, there has still been a loss in productivity and thus cost-effectiveness due to the need to repeatedly stop production to clean press platens or/or dies.
Thus, there is still a need for ways to reduce PMDI adhesion and reaction-product build up on press platens and masking of die surfaces.